The invention relates generally to the manufacture of electronic devices. More specifically, this invention relates to compositions and methods for trimming photoresist patterns useful in shrink processes for the formation of fine lithographic patterns.
In the semiconductor manufacturing industry, photoresist materials are used for transferring an image to one or more underlying layers, such as metal, semiconductor and dielectric layers, disposed on a semiconductor substrate, as well as to the substrate itself. Photoresist materials further find use, for example, in semiconductor manufacture in the formation of ion implantation masks. To increase the integration density of semiconductor devices and allow for the formation of structures having dimensions in the nanometer range, photoresists and photolithography processing tools having high-resolution capabilities have been and continue to be developed.
Positive-tone chemically amplified photoresists are conventionally used for high-resolution processing in optical lithography. Such resists typically employ a resin having acid-labile leaving groups and a photoacid generator. Exposure to actinic radiation causes the acid generator to form an acid which, during post-exposure baking, causes cleavage of the acid-labile groups in the resin. This creates a difference in solubility characteristics between exposed and unexposed regions of the resist in an aqueous alkaline developer solution. In a positive tone development (PTD) process, exposed regions of the resist are soluble in the aqueous alkaline developer and are removed from the substrate surface, whereas unexposed regions, which are insoluble in the developer, remain after development to form a positive image.
The theoretical resolution limit as defined by the Rayleigh equation for optical imaging tools is shown below:
  R  =            k      1        ⁢          λ      NA      where k1 is the process factor, λ is the wavelength of the imaging tool and NA is the numerical aperture of the imaging lens. The theoretical resolution limit for printing line and space patterns for standard 193 nm dry lithography, assuming NA=0.9 and k1=0.25, is 54 nm. This means that a 193 nm dry immersion tool would only be capable of resolving 54 nm half-pitch line and space patterns. For 193 nm immersion lithography (NA=1.35, k1=0.25), the theoretical resolution limit is reduced to 36 nm. The resolution for printing contact holes or arbitrary 2D patterns is further limited due to the low aerial image contrast with a dark field mask wherein the theoretical limit for k1 is generally higher than that for line and space patterns.
To form finer photoresist patterns than attainable by direct imaging alone, photoresist pattern trimming processes have been proposed. U.S. Pat. No. 6,492,075B1, for example, discloses a method of trimming a patterned resist. The method involves the steps of: providing a patterned resist having structural features of a first size, the patterned resist containing a polymer having a labile group; contacting a coating containing at least one cleaving compound to form a thin deprotected resist layer at an interface between the patterned resist and the coating; and removing the coating and the thin deprotected resist layer leaving the patterned resist having structural features of a second size that is smaller than the first size. This document discloses laundry lists of various cleaving compounds chosen from acids, bases, and organic compounds, and a number of possible polymers for use in the trimming compositions.
There is a continuing need in the art for improved lithographic methods useful in electronic device fabrication that provide increased resolution over that obtained by direct optical imaging which are capable of forming fine lithographic patterns with controllably reduced resist pattern dimensions.